1. Field of the Invention
The present invention relates to the field of fluid sampling devices and, more particularly, to a device and method for sampling ultra-pure chemicals.
2. Prior Art
Various devices and techniques are well-known in the prior art for sampling fluids, especially liquids. The most primitive of these techniques is the lowering of an open sampling container into the liquid and withdrawing the container once it is filled with a sample of the liquid. A refinement to this simple sampling container is an apparatus which encloses and confines the sampling liquid once the sample is taken. Typically, these type of containers are lowered into the liquid and once the container holds the sample, the container is "capped" in order to retain the liquid. This technique also prevents the sample from being contaminated as the container is withdrawn from the liquid. A device utilizing this technique is described in U.S. Pat. Nos. 4,305,279 and 4,852,413.
In most instances, a significant number of these devices are utilized for sampling water. When sampling water, it is not critical to preserve the sample in an ultra clean environment. The intent of maintaining sample integrity is typically of concern when the water sample is being withdrawn through a confined area which could introduce contaminants into the sample.
However, when liquids other than water are concerned, maintaining purity of the sample may be a critical concern as the sample is removed from the liquid source. It is imperative to maintain the purity of the sample to reflect the actual condition of the liquid being sampled. It is also imperative that the liquid source is not contaminated during the sampling procedure. Once such apparatus and method for withdrawal of liquid samples from a sterile liquid source is described in U.S. Pat. No. 4,999,307. Furthermore, various other closed systems are well-known in the prior art for taking samples from a liquid source in a confined system. For example, U.S. Pat. No. 3,922,920 describes a technique for providing the capture of a fluid sample from a fluid source, utilizing a chamber whose metering portion holds a pre-set volume of fluid. U.S. Pat. No. 4,213,342 describes a sampling device which samples by the use of a connected source of pressure to cause movement of sampled liquid in and out of a sealed container.
Although the art is populated by devices and techniques for taking samples from a liquid source, the field is narrowed considerably when samples are to be taken from ultra-pure chemicals which may be hazardous to the human operator. Further limitation is encountered when such sampling devices are to be made portable.
For example, in the processing of semiconductor wafers, ultra-pure chemicals are necessary to process semiconductor wafers to fabricate integrated circuit (IC) devices. The semiconductor industry dictates the use of the purest chemicals made. The chemicals used include concentrated acids and bases, which can degrade many of the commercially available sampling devices. Typically these chemicals are required to have less than 10 parts per billion (ppb) of most metals. However, due to the ever improving technology of IC fabrication, metal contaminants will need to be reduced to 1 ppb or less. This requirement will dictate that any method employed to obtain a representative sample be able to do so without contaminating the chemical. Thus, samplers made from metals, borosilicate glass and polyethylene cannot be used.
The ultra-pure chemicals are typically received in "drums" from a supplier of the chemical and the contents are typically tested prior to usage, such as during "receiving inspection". In this instance, a human operator will take samples from each of drums and remove those samples to a central testing facility, such as a chemical laboratory, and determine the purity of the chemical under inspection. Therefore, a number of stringent constraints are placed on the device and technique utilized for taking such samples.
The sampling apparatus must be portable and must not present undue burden to the operator in transporting and manipulating the apparatus. Furthermore, the apparatus must be capable of taking representative samples from the drum while maintaining a substantially clean environment in order not to contaminate the liquid source or the sample being taken. The collected sample must be maintained in an environment which is not susceptible to contamination while being transported back to the laboratory. Finally, the sampling apparatus and technique must not cause the operator to be subjected to the toxic or hazardous chemicals and fumes during the sample taking procedure.
Two of the better chemical sampling devices known in the prior art are described below. A chemical sampling kit utilizing a syringe type head for drawing samples from acid sinks and large canisters into a clean container is one of the simpler portable devices used in the testing of chemicals. One such chemical sampling kit is sold by Balazs Analytical Laboratory of Sunnyvale, California. In another apparatus, a peristaltic pump is utilized. Instead of the use of a syringe type head, a peristaltic pump is used to pump the liquid from the drum in order to draw the sample. Other simpler tools are also being used currently to draw liquid chemical samples from the drum. Such tools are commonly referred to as coliwasas, bailers, and grab samplers. Typically, the use of these devices requires that the drum containing the chemical be left opened to the environment during the sampling process, which exposure is more than sufficient to contaminate the drum.
Although these various prior art devices may function adequately in taking samples from chemical containers, they present a number of disadvantages (or short comings) to the semiconductor industry as requirements for chemicals having low levels of contaminants are needed. The present invention provides an apparatus and technique in order to meet the needs of taking samples of ultra-pure chemicals, particularly when these chemicals are for use in IC fabrication.